The present invention relates to a method and also a device for selectively determining the content quantities of gases, such as carbon dioxide, nitrogen and/or oxygen, for example, dissolved in liquids, such as beverages, and also for ascertaining the solubility (solubilities) and/or saturation pressure(s) thereof in the liquids.
The main emphasis of the invention is the determination of the content quantities of gases dissolving in liquids or dissolved in the same in fairly large quantities. However, as well as the content quantities of more than one gas dissolved in a liquid, which is of prime interest here, it should also be possible to determine the solubility (solubilities) and/or the saturation pressure(s) of an individual gas or of several gases in the liquid.
Constituting a further essential subject of the invention is a device in the form of various design variants for determining content quantities, solubility (solubilities) and/or saturation pressure(s) of at least one gas, but preferably at least two or more gases, in a liquid containing the gas or gases dissolved therein, the main emphasis being the implementation of the new determination method just referred to.
A considerable number of often quite different methods and devices for determining the content quantities of gases dissolved in liquids, such as carbon dioxide in particular, have become well known and also commercially available. Some of these known methods and devices are in principle also suitable for determining other gases, such as oxygen and/or nitrogen in particular, dissolved in liquids such as beverages, for example. The characteristic features of these will be briefly described below:    a) Pressure and temperature measurement in a sealed measuring chamber expanded once:            A representative sample of the liquid for measuring is introduced into a measuring chamber. Where measurements are performed on casks of beverages, the whole cask often serves as the measuring chamber. After the measuring chamber has been closed, the liquid sample to be analyzed is expanded by increasing the volume of the measuring chamber, e.g. by means of a piston-type injector fitted fluid-tight to the chamber, or, where measuring is being implemented in the beverage cask, by means of a short release of pressure. The pressure established after the expansion and the sample temperature are then measured. The carbon dioxide content is then calculated from these in accordance with Henry's law. Other gases dissolved in the sample liquid, such as oxygen and nitrogen in particular, affect the ascertained carbon dioxide content. Commercially available devices differ from one another by, among other things, the method of sampling, the shape of the measuring chamber, and by differing measures for the accelerated establishment of equilibrium pressure after the expansion or for extrapolating the equilibrium pressure.            b) Measurement in a liquid-free measuring chamber separated by a gas-permeable membrane:            A membrane substantially permeable only to carbon dioxide separates a measuring chamber from the sample liquid. The measuring chamber is periodically evacuated or flushed with a reference gas. The carbon dioxide content or its variation over time in the measuring chamber is then ascertained by measuring pressure, thermal conductivity or infrared absorption and in addition temperature, and from these the carbon dioxide content of the liquid is then calculated.            c) Measurement in a liquid-filled measuring chamber separated by a gas-permeable membrane:            A membrane substantially permeable only to carbon dioxide separates a measuring chamber filled with a suitable liquid from the liquid for analysis. As a result of absorbing the carbon dioxide permeating through the membrane, the liquid in the measuring chamber changes, e.g. in its pH value, which is measured together with the temperature, from which the carbon dioxide content of the liquid to be analyzed for the carbon dioxide content can then be calculated.            d) Direct infrared absorption measurement:            By means of infrared absorption measurements, usually in the mid infrared range, performed on the liquid to be analyzed, or sample liquid, the carbon dioxide content in the liquid is directly determined.            e) Wet chemical analysis:            In a defined sample volume, by means of the addition of appropriate chemicals the dissolved carbon dioxide is absorbed, separated out, and determined gravimetrically or titrimetrically.        
In d) and e) the carbon dioxide content is directly determined; in a)-c) the saturation pressure of the carbon dioxide in the liquid for analysis is primarily ascertained. With the solubility of the carbon dioxide in the liquid for analysis taken as known, assumed or only estimated, the carbon dioxide content of this liquid is calculated from the saturation pressure ascertained directly or indirectly. Since in practice the solubility of a gas in an aqueous solution other than pure water is always known only approximately at most, a problem arises from this if the results of different methods are compared with one another.
The present invention is based in particular on the group of methods and devices described above under a) which are based on pressure and temperature measurements in a sealed measuring chamber which has been expanded in a defined manner in only one step.